By RANDAL C. ARCHIBOLD

Published: January 29, 2007

At twilight on Friday, in the heart of the territory of the latest notorious Los Angeles gang, a woman in a passing car calls out a tip to Officer Dan Robbins, sending him racing toward a corner and a man he believes is a member of the 204th Street gang.

As Officer Robbins's black-and-white patrol car speeds forward, the man, Jose Covarrubias, 20, turns away and drops what appears to be a small pipe.

''Come here! Get your hands up!'' Officer Robbins of the Los Angeles Police Department shouts as he jumps out of the car and handcuffs Mr. Covarrubias, arresting him on suspicion of possessing drug paraphernalia, a methamphetamine pipe.

''You arresting all the black people here on Harvard Boulevard, too?'' Mr. Covarrubias asks, now seated on a curb, making plain the racial tension in this neighborhood, Harbor Gateway, that has commanded the city's attention.

They know each other, this gang unit officer and Mr. Covarrubias, who Officer Robbins says is a relatively new member of 204th Street, a Latino gang that gained notoriety last month when two members were charged in what the police said was the racially motivated killing of a 14-year-old black girl, Cheryl Green.

The crime stunned the city as a sign of growing violence among blacks and Latinos in some struggling neighborhoods and brought renewed promises from the mayor, the police chief and the F.B.I. director to reverse a surge of gang violence. They have promised more officers chasing the worst gang members, more school and community counselors and more cooperation among agencies.

In the department's Harbor Division, far from the worst in gang crime but the focus of political and news media attention since the killing, officers have started joint patrols with other police agencies. A deputy city attorney, Panagiotis Panagiotou, has ridden with Officer Robbins for part of his shift in an effort to broaden prosecutors' gang knowledge.

Crucial to the effort are the knowledge and wherewithal of gang unit officers like Officer Robbins, whose focus is tracking gangs operating in and near Harbor Gateway, a compact 12-square-block collection of apartment houses and single-family homes in a narrow sliver of Los Angeles 20 miles from downtown.

Officer Robbins, 36, has been on the force for 12 years, the last two with the Harbor Division gang unit.

He embodies in many ways the newest incarnation of the gang enforcement detail, which has a storied but troubled past.

Los Angeles has long been a model for other cities in gang enforcement. Police officials from across the country and Latin America will gather here on Feb. 7 to share information and strategize. Chief William J. Bratton, visiting Washington this week, plans to meet with members of Congress and federal officials to advocate for more sharing of intelligence on gangs, terrorists and organized crime groups.

Yet as much as gang crime has bedeviled Los Angeles, so has controversy over tactics to fight it, most notably a scandal in the Rampart Division that came to light in 1999. A gang unit officer who had been charged with stealing cocaine from an evidence locker said officers had beaten, shot and framed innocent people, leading to the reversal of scores of convictions and $70 million in legal settlements.

The police abolished an elite antigang unit known as Crash, and a federal consent decree was imposed requiring a more stringent system of checks and balances among gang enforcement units.

The effect of that order is immediately evident in Officer Robbins's attire. He wears a standard uniform and drives a marked squad car, albeit without lights on top, making him and other gang officers more visible, for better or worse.

Gang officers also generally serve in the new units, known as Gang Impact Teams, or GIT, for three years, with extensions requiring layers of approvals.

''We do lose some of the institutional knowledge,'' said Lt. Roger Murphy, who commands the Harbor Division's gang officers. ''It takes two to three years to get to know a gang. But we have to follow these strict protocols.''

With a reporter and photographer allowed to accompany him for several hours of his nearly 12-hour shift, Officer Robbins offered a primer on the 204th Street gang.

He knows its history, how it broke long ago from Tortilla Flats, now a bitter rival.

He knows its numbers: more than 100 members, with about 30 hard-core devotees, relatively small for a Los Angeles gang, some of which number into the thousands.

He knows that it earns money dealing marijuana and methamphetamine, but that the income is not enough for several members, who have legitimate day jobs.

He knows that members of 204th Street dislike blacks, who are the newer arrivals in the neighborhood, and he has learned from members that their disdain arises from racial hatred and rivalry with black gang members they accuse of dealing drugs on their turf.

And he knows Mr. Covarrubias, who has been arrested and jailed for burglaries and other offenses and has been a suspect in other gang-related crimes.